This invention relates to a process for estimating the inclusion content of metals. In addition, this invention can be used to estimate any physical property of a metal if the physical property is dependent on its inclusion content.
Metal producers often have the need to quantify inclusion content in metals. Inclusions include materials such as oxides, nitrides, sulfides, and other foreign matter which is present in the metal. For example, magnesium oxide is a common inclusion found in magnesium and magnesium alloys. Inclusion content is important because inclusions can lead to diminished physical properties of the metal, propagation of cracks, and ultimate failure of the metal.
The common methods for determining inclusion content are lengthy, time consuming and expensive. A typical method for determining inclusion content involves light microscopy on a polished and etched metal surface, also known as light optical metallography. Optical metallography requires multiple steps: sectioning, grinding, polishing, etching, and examination. Sectioning exposes an inner surface of the metal. Grinding flattens the exposed surface. Polishing makes the exposed surface scratch-free and is accomplished using electrolytic, chemical, or mechanical methods. Etching is accomplished using electrolytic or chemical methods in order to better reveal the microstructure of the sample. Examination typically occurs under a microscope, where the inclusions are counted and analyzed. An image analyzer can be used to count the inclusions. Inclusion content may be expressed as weight percent, volume percent, area percent, number per unit area or any other similar quantity.
The above technique is relatively expensive and is sometimes difficult to employ since the number of inclusions per unit area of a polished surface may be relatively small. On the other hand, a fracture often propagates along a path of inclusions. Thus, there are generally more inclusions visible on a fractured surface than on a flat polished surface. As a result, fractography, the study of fractured surfaces of metals, is another method of inclusion analysis. In fractography, a sample of metal is fractured, and the fractured surface is examined for inclusions. Inclusion content is usually expressed as the number of inclusions per unit area of fracture surface. Accurate fractographic examination occurs under a light optical microscope. Fractographic examination using a microscope is relatively expensive, slow and cumbersome.
Light optical metallography of polished metal surfaces and fractography are difficult to utilize in clean die casting metals where the inclusion content is relatively low. As a result, inclusion concentrating techniques are often used to facilitate analysis of such metals. Most commonly, molten metal is pulled through a porous ceramic filter, forming a cake of inclusions next to the filter, and the metal and filter cake are frozen with the ceramic filter contained therein. The filter cake of inclusions is then analyzed using light optical metallography as described above. Again, this technique is expensive, slow, and cumbersome.
Another method of measuring the inclusion content of metals that has gained some acceptance in the aluminum industry is the use of a Liquid Metal Cleanliness Analyzer (LMCA). In order to use a LMCA, a tube with a small non-conducting orifice is immersed in molten metal. As the molten metal flows through the small orifice, the LMCA electrically counts and sizes the non-metallic inclusions. However, the materials of construction are not suitable for some metals such as magnesium. In addition, the small orifice may become plugged if the metal contains relatively large inclusions. Furthermore, although this method is relatively fast, the measurement instrument is expensive.
Other less common methods for measuring the inclusion content of metals include selective dissolution techniques, neutron activation analysis, glow discharge mass spectrometry, arc-spark emissions spectrometry, distillation, and ultrasonic techniques. However, these methods are also relatively expensive and cumbersome.
Optical metallography, fractography and LMCA are thorough and precise processes. In addition to revealing the inclusion content of a metal, these methods reveal the sizes and shapes of the inclusions. However, metal manufacturers and recyclers are often interested in the inclusion content alone and are not concerned with the size or shape of the inclusions. The thoroughness of optical metallography, fractography, and LMCA is not necessary if inclusion content alone is desired.
Furthermore, as discussed above, these other methods of inclusion analysis are expensive and usually cumbersome procedures. It would be a further advance in the art of inclusion analysis to have a faster, simplified and less expensive instrumental method for determining inclusion content in metals.